Right, but with a lapbelt in the center, the boostered 7 yo can't be there, so the 3 yo has to be in the center using the lapbelt to install a harnessed seat (did you say that? I'm sure you must have...).
Rearfacing is SO much safer than forward facing, even harnessed, that even if the baby is more 'fragile', it is counteracted by being protected best in the rearfacing seat (the seat shell rotates toward the point of impact and will stay between the baby and the car door, somewhat, while a 3 yo's head will fly forward and out of the confines of the shell and be more likely to strike the interior of the car or the oncoming car's bumper...but is still more protected than the 7 yo in a booster...but there's no choice of truly optimal protection if there's only a lapbelt for the 7 yo, unless the parent uses a harness system like the ridesafer travel vest or ez-on vest).
It's kind of a judgement call, and frankly sometimes we can't always do the optimal scenerio (for instance, i can only fit my infant in between my older boostered kids, for family sanity reasons, lol)...but 'most protected child in the least protective spot' is "best practice" that is slow to catch on among even instructors.